Graded-Band-Gap Semiconductors: the Possibilities for Improvement of p-n Junction Performance
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M8.13.1
Graded-Band-Gap Semiconductors: the Possibilities for Improvement of p-n Junction Performance Bogdan Sokolovskii and Roman Yasnytskyi Ivan Franko National University, Lviv UA-79602, Ukraine ABSTRACT The paper theoretically analyzes properties of graded-band-gap (GBG) p-n structures in which band gap linearly enlarges with increasing the distance from the junction metallurgical edge. It is shown that by means of band-gap grading one can significantly reduce the diffusion reverse current caused by carrier thermal generation in the structure base regions and Ohmic contacts. The numerical estimations have been made for the case of p-n junction on the basis of CdHgTe solid solution. INTRODUCTION The spatial dependence of energy band-gap diagram occurring in GBG semiconductors results in appearance of quasi-electric fields which have great influence on carrier transport and give rise to a number of new effects [1]. Despite of sufficiently wide application of GBG materials in semiconductor electronics their unique properties do not revealed and used completely. In particular, of interest is the analysis of possibility of using the band-gap grading for improvement of p-n junction performance, first of all, the reduction of reverse current, which to great extend determines the ultimate characteristics of various p-n junction based devices, for example, IR detectors. The case of coordinate independent band-gap gradient [2] is not favorable for this aim, since the action of the quasi-electric fields on the minority carriers in the base regions located from the different sides of space-charge region (SCR) leads to the opposite effects in the respect of the reverse current associated with thermal generation of carriers. In this paper, we proved theoretically the possibility of significant suppression of the reverse current in the p-n structures with nonmonotonic band-gap profiles in the region of the junction location. THE MODEL OF p-n STRUCTURE AND BASIC EQUATIONS Let us consider a GBG structure whose regions -dp < x < 0 and 0 < x < dn are uniformly doped with acceptor and donor impurities at concentrations Na and Nd , respectively. The band gap Eg and the electron affinity χ are assumed to linearly vary with the co ordinate, with Eg reaching its minimum at the interface (x = 0) (figure la). With taking into account the carrier redistribution at the interface what results in the formation of SCR, the energy band-gap diagram of the p-n structure has the form presented in figure 1b. Under the presence of quasielectric fields the electron jn and hole jp current densities are given by the following expressions [3]:
M8.13.2
dn p
jn = kTµ n
dx
dpn
j p = − kTµ p
− γ pnp ,
dx
− γ n pn ,
(1)
where γ p = −(1 / kT )dE g / dx at x < 0, γ n = (1 / kT )dE g / dx at x > 0, np and pn are the concentrations of nonequilibrium electrons and holes in the p and n base regions; µn and µp are the electron and hole mobilities, assumed to be coordinate independent. It should be noted that field componen
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